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How does a Map differ from a plain object for storing key-value pairs? How does a Set differ from an array? How do you iterate over a Map's entries? When would you choose a WeakSet over a Set? How do you convert between a Map and a plain object? How would you use a Map to implement an LRU (Least Recently Used) cache? How do WeakRef and FinalizationRegistry extend the weak-reference capabilities beyond WeakMap/WeakSet? Why can't you use WeakMap for caching results keyed by primitive values like strings or numbers? How would you group an array of objects by a property using Map, and how does the newer Object.groupBy() compare?
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JavaScript › Map, Set, WeakMap & WeakSet

What is a WeakMap and why can't you iterate over its keys?

Intermediate

Answer

A WeakMap holds its keys weakly, meaning entries don't prevent the garbage collector from reclaiming a key object once there are no other references to it — this makes iteration (or even .size) impossible, since the set of live keys can shrink at any unpredictable moment via GC.
const cache = new WeakMap();
let obj = { id: 1 };
cache.set(obj, 'metadata');
obj = null; // the entry can now be garbage collected automatically
Real-world example Attaching private metadata to DOM elements without preventing them from being garbage collected when removed from the page.

Common follow-ups: What restrictions does WeakMap place on what can be used as a key?

Memory Management & Garbage Collection
All 10 Map, Set, WeakMap & WeakSet questions